As a technique drawing attention as a backbone construction technique for the next generation IP network (NGN: Next Generation Network), there is RPR (Resilient Packet Ring). RPR is standardized as IEEE 802.17. RPR is a technique for efficiently using a high-speed line of 10 Gbit/sec on a per-frame basis. In IEEE 802.17, a ring topology and a cascade topology are defined as connection modes of nodes.
FIG. 1 is a diagram illustrating an example of the ring topology. The ring topology is a mode of circularly connecting nodes. With the ring topology, even if a failure occurs, communication is possible in the direction opposite to the failure, and the reliability is high. Accordingly, with the RPR ring topology, recovery within 50 ms of occurrence of a failure, which is an indication of availability of a communication service provided by a telecommunication carrier, may be achieved. However, due to the specifications of IEEE 802.17, there is a constraint that, in the case of constructing a ring topology, all the transmission paths on the ring have to be in accordance with RPR.
FIG. 2 is a diagram illustrating an example of the cascade topology. The cascade topology is a mode of serially connecting nodes. With the cascade topology, it is highly likely that the network becomes partitioned due to a transmission path failure at one portion, and communication becomes disabled.